16DC – DC Converter Fed Drives - Two-quadrant ControlFor fast transition from motoring (Q1) to braking (Q2) and vice versa, both T1 and T2 are controlled simultaneously, i.e. within a period T:T1 in ON and T2 is OFF between time 0 < t ≤ tonIf Ia is positive (Va > E), current flows from supply to motor via T1If Ia is negative (E > Va), current flows from motor to supply via D1T1 is OFF and T2 is ON between ton < t ≤ TIf Ia is positive, current circulates via D2If Ia is negative, current circulates via T2Duty ratio is given by:Average armature voltage is:Average Va =VDr. Ungku Anisa, July 2008EEEB443 - Control & Drives

18DC – DC Converter Fed Drives - Four-quadrant ControlOperation in all four quadrantsVa and Ia can be controlled in magnitude and polarityPower flow can be in either directionSpeed and torque can be reversedTQ1Q2Q3Q4+ Va -T1D1T2D2D3D4T3T4iaNote:Polarity of Va and direction of Ia indicated are assumed positive.Dr. Ungku Anisa, July 2008EEEB443 - Control & Drives

19DC – DC Converter Fed Drives - Four-quadrant ControlWhen a switch is on (i.e. ‘ON state’) it may or may not conduct current depending on the direction of iaIf a switch conducts current, it is in a conducting stateConverter has two legs (Leg A & Leg B)Both switches in each leg, are alternately switchedIf T1 = ON, T4 = OFFIf T4 = ON, T1 = OFFLeg B+ Va -T1D1T2D2D3D4T3T4+Vdc-iaDr. Ungku Anisa, July 2008EEEB443 - Control & DrivesLeg A